What is the difference between knowingly receiving stolen property and not knowing but finding out later and doing nothing about it?

In both instances, one could be charged with receiving stolen property.

Knowingly means with knowledge; consciously; intelligently; willfully; intentionally.

Whether one knew the property was stolen when receiving it or discovered later that the property was stolen and did nothing about it, one had knowledge that the property was stolen and therefore, could be charged with receiving stolen property.  Knowledge refers to actual knowledge.  So, when an individual subsequently discovers that property received was stolen and does nothing about it, the individual has actual knowledge that the property was stolen, and by doing nothing about it is still acting willfully and intentionally to keep the stolen property.  A criminal act requires both the physical act and the requisite mental state.  The physical act is receiving the stolen property and the requisite mental state is doing so knowingly.  Knowingly is a perception of facts to constitute a crime.  A knowledge that facts exist (property was stolen) bring the act or omission within provisions of the Code (receiving stolen property in this case).